1. Field of the Invention
This invention relates to an automatic focusing device for a video camera, an electronic camera, etc. arranged to automatically perform a focusing action using a video signal produced from an image sensor or the like.
2. Description of the Related Art
Automatic focusing devices of varied kinds have been known. For a video camera, an electronic camera or a like apparatus of the kind using an image sensor, there is known a method of focusing by detecting the sharpness of the image of an object formed on an image sensing plane through a video signal obtained from the image sensor and by driving an optical system to a position where a maximum degree of sharpness is obtained. The sharpness of an object's image is determinable in principle according to the largeness or smallness of the high frequency component of an image signal. For example, the high frequency component of an image (or video) signal is extracted by means of a high-pass filter or a differentiating circuit. Then, a focal point is adjusted by driving a photo taking optical system until the high frequency component reaches a peak value. With regards to the automatic focusing device of the above stated kind, a method called "hill climbing servo method" which is described in detail in "NHK GIJUTSU KENKYU," 1965, Vol. 17, No. 1, (Whole No. 86), p 21 to p 37, has long been well known. A basic circuit arrangement of these prior art methods is as shown in FIG. 1 of the accompanying drawings. A gate circuit 12 is arranged to extract, from an image signal which is produced from an image sensor 10, a signal corresponding to a given area (a middle area of an image plane in general) set for detection of the focused state of the image plane. A high-pass filter 14 is arranged to extract a high frequency component of the signal output of the gate circuit 12. A peak detecting circuit 16 is arranged to detect the peak value of the high frequency component of the image signal for the image plane (or one frame or one field). A direction determining circuit 18 is arranged to determine the shifting direction of an image sensing optical system 20 by comparing the peak value detected by the peak detecting circuit 16 with the peak value of the previous image plane. The direction determining circuit 18 causes, by controlling the motor driving circuit 22, a motor 24 to rotate in the same direction as before if the detected peak value is found to have increased from the previous peak value and to rotate in the reverse direction if the peak value is found to have decreased from the previous peak value. In the event of no substantial change, in the value, the motor 24 is caused to stop from rotating.
The above stated conventional method, however, has presented the following problems: First, since the focus detecting area is set at a fixed part on the image plane, the detected value of the high frequency component tends to fluctuate to a great degree as the image is caused to move into or out of the focus detecting area by the shaking of hands or by the movement of the object to be photographed. Then, the optical system would be driven in a wrong direction. The optical system would fail to come to a stop and fluctuate when an in-focus stat is obtained. Secondly, some images other than a desired object's image commingle with the latter within the focus detecting area. If such other images include, for example, a flickering lamp or the like, an erroneous focusing action also would result from such a condition.
To eliminate the possibility of such an erroneous action, an automatic follow-up detecting device and an automatic follow-up device have been disclosed in U.S. patent application Ser. No. 737,163, filed May 23, 1985, assigned to the same assignee as the assignee of the present invention. The devices disclosed are arranged to store the features of an object (or a main photographing object); to detect and incessantly follow-up the movement of the object; and to be capable of setting a focus detecting area always appositely to the position of the object. In accordance with this method, the focus detecting area is arranged to follow the movement of the object. The method, therefore, prevents any erroneous action that results from shaking unsteady hands, a move of the object or a coexisting object which is not desired.
However, since the movement of the object is followed generally by using information on an image signal, the above stated method necessitates the use of a memory for storing a feature of the object such as a pattern of the object and a high-speed correlation computing circuit for computing the moving extent of the object. This prior art method thus has necessitated a large-scale, expensive circuit arrangement.